netdev - Re: [RFC v2] fq_codel : interval servo on hosts

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Date:	Tue, 4 Sep 2012 09:40:40 -0700
From:	Dave Taht <dave.taht@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>, codel@...ts.bufferbloat.net,
	Kathleen Nichols <nichols@...lere.com>,
	netdev <netdev@...r.kernel.org>, Tomas Hruby <thruby@...gle.com>
Subject: Re: [RFC v2] fq_codel : interval servo on hosts

On Tue, Sep 4, 2012 at 8:34 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2012-09-04 at 08:10 -0700, Nandita Dukkipati wrote:
>> The idea of using srtt as interval makes sense to me if alongside we
>> also hash flows with similar RTTs into same bucket. But with just the
>> change in interval, I am not sure how codel is expected to behave.
>>
>> My understanding is: the interval (usually set to worst case expected
>> RTT) is used to measure the standing queue or the "bad" queue. Suppose
>> 1ms and 100ms RTT flows get hashed to same bucket, then the interval
>> with this patch will flip flop between 1ms and 100ms. How is this
>> expected to measure a standing queue? In fact I think the 1ms flow may
>> land up measuring the burstiness or the "good" queue created by the
>> long RTT flows, and this isn't desirable.

Experiments would be good.

>
> Well, how things settle with a pure codel, mixing flows of very
> different RTT then ?

Elephants are shot statistically more often than mice.

> It seems there is a high resistance on SFQ/fq_codel model because of the
> probabilities of flows sharing a bucket.

I was going to do this in a separate email, because it is a little off-topic.

fq_codel has a standing queue problem, based on the fact that when a
queue empties, codel.h resets. This made sense for the single FIFO
codel but not multi-queued fq_codel. So after we hit X high rate
flows, target can never be achieved, even straining mightily, and we
end up with a standing queue again.

Easily seen with like 150 bidirectional flows at 10 or 100Mbit.

(as queues go, it's still pretty good queue. And: I've fiddled with
various means of draining multi-queue behavior thus far, and they
ended up unstable/unfair)

> So what about removing the stochastic thing and switch to a hash with
> collision resolution ?

Was considered and discarded in the original SFQ paper as being too
computationally intensive
(in 1993). Worth revisiting.

http://www2.rdrop.com/~paulmck/scalability/paper/sfq.2002.06.04.pdf

>
>



-- 
Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
with fq_codel!"
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